U.S. military officials say pro-Assad regime forces are gathering east of the Euphrates River in Syria near where Assad forces attacked U.S.-backed forces in February, causing a massive loss of life of regime troops and Russian mercenaries, CNN reported Thursday.

"While we do not seek confrontation with pro-regime forces, as we have demonstrated repeatedly, Coalition forces will not hesitate to protect themselves when they are threatened," Pentagon spokesman Maj. Adrian Rankine-Galloway told CNN.

During the February attack pro-regime forces using tanks and artillery fired on a Syrian Democratic Forces headquarters where U.S. Special Operations Forces were present.

A U.S. official claimed more than 100 fighters aligned with Syrian President Bashar al-Assad died when coalition and local coalition-backed forces thwarted a large attack overnight on February 7. However, other local reports place the death toll as high as 200.

Russia's Defence Ministry, which supports Assad's forces in the Syrian civil war, said at the time that pro-government militias involved in the incident had been carrying out reconnaissance and no Russian servicemen had been in the area.

The US-led coalition later said that the February attack by forces aligned with Assad was likely intended to seize nearby lucrative oil fields that US-backed forces had captured from ISIS.

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